A laser surveying instrument is used to project a laser beam by rotary irradiation and to form a reference line or a reference plane necessary for construction work and civil engineering work. For the purpose of forming a horizontal reference plane and setting a tilted reference plane, the laser surveying instrument has a tilt detecting device.
In the past, laser surveying instruments have been known, which can form a horizontal reference plane and can form a reference plane with a desired tilt angle as disclosed in the Patent Publication JP-A-2007-132716 (the Patent Document 1) and in the Patent Publication JP-A-2006-308592 (the Patent Document 2).
The tilt detecting device as disclosed in the Patent Document 1 is a combination of a tilt sensor for detecting horizontality with high accuracy and an encoder for detecting a tilt angle from a reference position. The tilt sensor detects horizontality, and the encoder detects an angle from the horizontality with high accuracy. The tilt sensor detects horizontality with high accuracy and the encoder detects an angle from the horizontality as set by the tilt sensor, and an arbitrary tilt angle from the horizontality can be detected.
The tilt detecting device as disclosed in the Patent Document 2 is a combination of a tilt sensor for detecting the horizontality with high accuracy and a gravity sensor for detecting a tilt angle with respect to the gravitational field. The horizontality is accurately detected by the tilt sensor, and tilt angle is directly detected by the gravity sensor.
By the tilt detecting device as disclosed in the Patent Document 1, an angle can be detected with high accuracy by the encoder. Thus, by the laser surveying instrument disclosed in the Patent Document 1, a horizontal reference plane and a tilted reference plane can be set with high accuracy.
However, in the tilt detecting device disclosed in the Patent Document 1, there are problems that the mechanism of the encoder is complicated and that higher cost is required. In the tilt detecting device as disclosed in the Patent Document 2, the horizontality can be detected with high accuracy, but the tilt angle is detected by using the gravity sensor, and the gravity sensor has lower resolution (lower detection accuracy). For this reason, the tilt detecting device according to the Patent Document 2 has simple structure, but there is a problem in that the detection accuracy to detect the tilt angle is low.
To overcome the above problems, it is an object of the present invention to provide a tilt detecting device, which has simple structure and can be produced at lower cost, and which can detect the tilt angle with higher accuracy. Also, the invention provides a laser surveying instrument produced at lower cost, which can set a horizontal reference plane or a tilted reference plane with high accuracy by adopting the tilt detecting device as described above in the laser surveying instrument.